HIV infections in sub-Saharan Africa increasingly occur among injecting drug users (IDUs), a most-at-risk population (MARP). Evidence-based services for IDUs such as needle and syringe exchange programs (NSPs), opioid substitution therapy (OST), and IDU-specific antiretroviral therapy (ART) adherence support have been non-existent in this region. Kenya is conducting size estimations of IDUs and is preparing to launch first-ever NSPs. Our team of Kenyan policy leaders, addiction/behavioral scientists and modelers will leverage Kenya's new MARPs/NSP platform to seek out IDUs, deliver rapid HIV testing, point of care CD4 count and link to ART, and evaluate community viral load in Nairobi and coastal Mombasa, where most IDUs in Kenya reside. Aim 1: Evaluate seek test treat retain - 'Testing &Linkage to Care for IDUs'(TLC-IDU Kenya) - using a stepped wedge cluster-randomized design. Clusters will be the planned n=20 MARP service sites and n=5 NSPs. We will initiate respondent-driven sampling (RDS) to reach IDUs in Nairobi and Mombasa for baseline HIV-1 prevalence determination, and then collect seven waves of study data as service sites roll out, including behavioral data on PDAs. Teams will do rapid HIV testing and refer for addiction/mental health and OST. HIV-positives will receive prevention with positives (PwP) counseling and point of care CD4 counts. Those with CD4 <350/<L will be assigned a peer case manager to link the person to ART at study-participating HIV clinics, support ART and PwP adherence and care retention. Both peer case managers and subjects will receive small conditional cash transfers for subject's adherence to HIV care visits. Primary study outcomes will include time to successful linkage to care, time to ART, and community viral load before and after the TLC-IDU initiation. 'Community viral load'will be ascertained by collecting specimens from n=10 randomly-selected HIV-positives at each of n=25 sites. This sampling will be done in eight waves over time, to document changes in infectivity (median viral load). With seven individual viral loads per site per time step (n=1400) we will have good power to detect log10 viral load changes of 0.23 and hazard ratios of ~1.5 when comparing pre- and post-intervention period using linear mixed effects analysis. Aim 2: Conduct mathematical modeling to estimate community viral load in IDU injecting and sexual networks, and to assess potential population-level impact of the TLC-IDU intervention on Ro, numbers of infections averted, and quality-adjusted life expectancy. Aim 3: Assess the incremental cost-effectiveness ratio of the TLC-IDU model, using a national payer perspective. This study will provide among the world's first data regarding implementation of the seek, test, treat and retain paradigm with IDUs in sub-Saharan Africa. It will demonstrate the degree to which a combination of structural, biomedical and behavioral interventions can reduce infectivity. Partnership with Kenya's national HIV program will allow lessons learned from this study to inform other countries considering how best to address the growing IDU contribution to the HIV epidemic in this high-HIV-burden region. PUBLIC HEALTH RELEVANCE: Interventions for injecting drug users (IDUs) in sub-Saharan African have been almost entirely absent, despite the fact that in countries like Kenya they contribute a growing proportion of incident HIV infections. This study will leverage a historic needle exchange program (NSP) for this most-at-risk population (MARP) in Kenya to seek out IDUs, deliver rapid HIV testing, point of care CD4 count and link to ART using peer case managers and evaluate community viral load impact using a stepped wedge cluster-randomized design. Lessons learned will have important applicability throughout sub-Saharan African.